This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/GB98/02505 which has an International filing date of Aug. 20, 1998, which designated the United States of America.
1. Field of the Invention
This invention relates to applanating tonometers for the measurement of intraocular pressure (IOP).
2. Description of Related Art
Known applanating tonometers employ a transparent applanating element with a contact face through which light can be transmitted onto, and be reflected from, the eye of a subject in a manner which varies with the degree of applanation. The aim is to determine the force required to put the contact face in contact with the cornea of the eye to applanate a given area and so provide a measure of the IOP.
One example of such a tonometer is described in GB 862920, which is a version of the instrument known as the Goldman tonometer. In order to standardize the area of applanation and avoid the need for pre-calibration, which-brings its own problems as explained in that document, the eye is observed through an applanating element that comprises a split prism. When the element is applied against the eye, the area of applanation can be observed through the split prism as two semi-circular images displaced relative to each other by an amount determined by the parameters of the prism. As the force of applanation and the applanated area increases, the images increase in size and the object observed when the half images come together, are seen as a continuous S-shaped line, when the image diameter equals the displacement of the images. This dimension is standardized, for practical reasons, at 3.06 mm.
However, the images produced do not allow a precise and unambiguous determination of the coming together of the images. A degree of subjective judgement is needed and there can therefore be significant variations in the readings taken. The readings are also dependent on the skill of the user, because it is not easy to ensure that the applanating element is always applied squarely to the cornea. Any tilt of the contact face relative to the eye will introduce measurement errors.
It would therefore be desirable to provide an instrument that could be operated with less dependence on the user""s skills and judgement in order to provide a more objective reading of intraocular pressure.
According to one aspect of the present invention, there is provided a tonometer comprising an applanating element having a light-transmitting contact face engageable against the eye of a subject for projecting a light beam onto the eye and passing reflected light therefrom. There are also means responsive to the reflected illumination and means responsive to the force of engagement of the applanating element on the eye for obtaining data of changing values of reflected illumination and force as the element is applied to the eye. The contact face has an area larger than a predetermined area of engagement at which the applanting force is to be determined. The instant of applanation of the predetermined area is obtained by interpolation. The cornea is applanated over the contact face area, dependent upon the ratio of the predetermined and larger areas of applanation, in order to determine the force measured at the instant of applanation of the predetermined area.
Preferably, means are provided for processing progressive measurements of the force and reflected illumination to derive a measure of acceleration of the applanating element at the moment of applanation, and preferably automatically correcting the applanating force measurement accordingly, in order to compensate for dynamic force components that may appear in the measurement of the force on the applanting element. The readings obtained can thereby be rendered independently, or at least less dependent, of any variations in the rate of application of the contact face against the cornea. In this way the use of the tonometer as a hand-held instrument can be facilitated without compromising the accuracy of measurement.
In a further aspect, the invention also provides a method of measuring intraocular pressure in which a contact face of an applanating element is applied against the eye of a subject and measurements are made that are indicative of both the degree of applanation and the force applied to the applanating element. The measurements are made while increasing the pressure of application until the contact face is fully applied against the eye to produce a predetermined lesser extent of applanation by the element by interpolation of the measurements of force.
According to another aspect of the invention, there is provided a tonometer comprising an applanating element having a contact face engageable against the eye of a subject and having means for determining the alignment of the contact face to the eye. The means comprising a light source directing a beam onto a beam-splitting element which transmits a first part of the beam onto a first reflecting element and reflects a second part of the beam onto a second reflecting element. Light from the first part of the beam is arranged to be reflected by the beam splitting element to the contact face and light from the second part of the beam being arranged to be transmitted through the beam-splitting element to the contact face. The positions of the images of the light from the first and second parts of the beam falling on the retina of the eye and are thereby dependent upon the relative alignment between an optical axis of the tonometer and the optical axis of the eye.
In such a tonometer, it can be arranged that the two reflecting elements are provided by front faces of a light-emitting device for producing an applanation measurement beam and a light-receiving device for reflected illumination from the eye of the measurement beam to derive a measure of the applanation pressure.
Conveniently, the elements can provide conjugate paths for the light transmitted for the light source to their respective reflecting surfaces. If the reflecting surfaces are located symmetrically to the optical axis of the tonometer, it can be arranged that by bringing the observed images into coincidence with each other will indicate that alignment has been achieved.